A growing number of high-performance papermaking machines with speeds of up to 2000 meters per minute and working widths above 10 meters are used in the paper production industry today. The sheet forming unit is very generally designed as a double cloth former, and in many cases as a split former. Typically, the sheet forming process takes place immediately between two papermaking wire cloths in a relatively short drainage zone. The time required for sheet formation is reduced to milliseconds by this short distance and the high production rate. Over this interval the solid fraction or dry content of the fiber suspension must be increased from about 1 percent to about 20 percent. For the papermaking wire cloth, this requirement means that this machine must be characterized by very high drainage performance, without leaving markings in the paper, and must also provide high fiber support.
Another important point is the transverse stability of wire cloth tension. This stability is of decisive importance in determining the thickness and moisture profile of the sheet. Very strict requirements are set in this connection, particularly for modern machines characterized by large operating widths. To improve sheet formation, shaping strips are consequently used with increasing frequency in the sheet forming zone. The shaping strips are mounted alternately on the backing sides of the wire cloth and are pressed against these sides. The result is rapidly alternating load deflection of the cover of the wire cloths in the longitudinal direction.
Today, an effort is customarily made to meet these requirements by use of composite fabrics. A composite fabric used for this purpose is described in DE 42 29 828 C2, for example. The conventional papermaking wire cloth in question has two superposed wire cloth fabrics in the form of single layers interconnected by binding threads extending in the cross and/or lengthwise direction. One of the fabrics is in the form of definition fabric having the mechanical properties of the composite fabric with respect to extension. The other wire cloth fabric in the form of reaction fabric of higher tension and lower rigidity than the definition fabric. The wire cloth fabrics include warp threads and woof threads connected to each other by additional binding threads. As a result of the design of the cloth fabrics as reaction or definition fabrics, internal wear, especially wear of the binding threads, is resisted. In this way, the service life of the combination fabric is lengthened, and undesirable separation of the wire cloth fabric layer is prevented over a long period. The internal wear of a combination fabric is caused especially by the circumstance that, during redirection of the wire cloth, such as occurs in the area of guiding rollers of the wire cloth batch by way of which the combination fabric is guided, the individual wire cloth fabric layers are stretched or crushed to varying degrees.
Since the binding threads not only belong to the structure of the fabric but are independent components, they are kept as small as possible in diameter in order to disrupt drainage as little as possible. With correspondingly high stresses, the possibility exists that the thin binding threads will then break, and the connection between the wire cloth fabrics will be interrupted. In the case of a generic papermaking wire cloth, as disclosed in EP 0 432 413 B1, which also has the structure of a composite fabric, the proposal has already been made that binding threads be used as two fabric-specific threads and interlaced with the other fabric layer involved to form X-shaped crossings in order to prevent the disadvantages in the state of the art described. The accumulation alone of the known change points in the cross-direction also results unintentionally in stiffening of the conventional fabric. Considerable differences in length may occur especially over greater weaving lengths which, in turn, are manifested in difference in tension, with the result that fabric-specific binding threads break and may result in failure of the conventional papermaking wire cloth. It is also known in connection with this conventional special type of weave that it is more or less possible to produce only cross-threads of one kind, that is, cross-threads of more or less the same diameter, for both the upper and the lower fabric, something which reduces the possibility of efficient support on the backing side. In addition, manufacture of the conventional interlocking fabric is costly.